Yuanbao Lin

A key challenge in the development of organic mixed ionic‐electronic conducting materials (OMIEC) for high performance electrochemical transistors is their stable performance in ambient. When operating in aqueous electrolyte, potential reactions of the electrochemically injected electrons with air and water could hinder their persistence, leading to a reduction in charge transport. Here, the impact of deepening the LUMO energy level of a series of electron‐transporting semiconducting polymers is evaluated, and subsequently rendering the most common oxidation processes of electron polarons thermodynamically unfavorable, on organic electrochemical transistors (OECTs) performance. Employing time resolved spectroelectrochemistry with three analogous polymers having varying electron affinities (EA), it is found that an EA below the thermodynamic threshold for oxidation of its electron polarons by oxygen significantly improves electron transport and lifetime in air. A polymer with a sufficiently large EA and subsequent thermodynamically unfavorable oxidation of electron polarons is reported, which is used as the semiconducting layer in an OECT, in its neutral and N‐DMBI doped form, resulting in an excellent and air‐stable OECT performance. These results show a general design methodology to avoid detrimental parasitic reactions under ambient conditions, and the benefits that arise in electrical performance.

A bromine‐substituted [2‐(9H‐Carbazol‐9‐yl) ethyl] phosphonic acid, 1Br‐2PACz, is designed as hole‐selective self‐assembled monolayers (SAMs), contributing to an outstanding power conversion efficiency (PCE) of 19.35% for binary bulk‐heterojunction (BHJ) based organic solar cells (OSCs). As compared to the previous high‐performance 2Br‐2PACz SAMs, 1Br‐2PACz molecules can effectively reduce the interaction of the SAM with the BTP‐eC9 nonfullerene acceptors with a decreased binding energy, resulting in the suppressed vertical self‐aggregation of BTP‐eC9 small molecules as the bottom side of PM6:BTP‐eC9 BHJ during the solidification process. There is decreased energetic disorder within photoactive layer together with more efficient charge transfer and suppressed non‐radiative recombination. Furthermore, five additional binary BHJ systems are applied in 1Br‐2PACz SAM‐based OSCs, exhibiting continuously superior performance as compared to the reference cells with conventional PEDOT:PSS hole transport layer. This work underscores the potential of advancing OSCs by fine‐tuning SAMs through halogenation strategies to improve active layer morphology and overall device performance.

ns² compounds have recently attracted considerable interest due to their potential to replicate the defect tolerance of lead-halide perovskites and overcome their toxicity and stability limitations. However, only a handful of compounds beyond the perovskite family have been explored thus far. Herein, we investigate bismuth sulfobromide (BiSBr), which is a quasi-one-dimensional semiconductor, but very little is known about its optoelectronic properties or how it can be processed as thin films. We develop a solution processing route to achieve phase-pure, stoichiometric BiSBr films (ca. 240 nm thick), which we show to be stable in ambient air for over two weeks without encapsulation. The bandgap (1.91 ± 0.06 eV) is ideal for harvesting visible light from common indoor light sources, and we calculate the optical limit in efficiency (i.e., spectroscopic limited maximum efficiency, SLME) to be 43.6% under 1000 lux white light emitting diode illumination. The photoluminescence lifetime is also found to exceed the 1 ns threshold for photovoltaic absorber materials worth further development. Through X-ray photoemission spectroscopy and Kelvin probe measurements, we find the BiSBr films grown to be n-type, with an electron affinity of 4.1±0.1 eV and ionization potential of 6.0±0.1 eV, which are compatible with a wide range of established charge transport layer materials. This work shows BiSBr to hold promise for indoor photovoltaics, as well as other visible-light harvesting applications, such as photoelectrochemical cells, or top-cells for tandem photovoltaics.

Developing efficient and stable organic photovoltaics (OPVs) is crucial for the technology's commercial success. However, combining these key attributes remains challenging. Herein, we incorporate the small molecule 2-((3,6-dibromo-9H-carbazol-9-yl)ethyl)phosphonic acid (Br-2PACz) between the bulk-heterojunction (BHJ) and a 7 nm-thin layer of MoO₃ in inverted OPVs, and study its effects on the cell performance. We find that the Br-2PACz/MoO₃ hole-extraction layer (HEL) boosts the cell's power conversion efficiency (PCE) from 17.36% to 18.73% (uncertified), making them the most efficient inverted OPVs to date. The factors responsible for this improvement include enhanced charge transport, reduced carrier recombination, and favourable vertical phase separation of donor and acceptor components in the BHJ. The Br-2PACz/MoO₃-based OPVs exhibit higher operational stability under continuous illumination and thermal annealing (80 °C). The T₈₀ lifetime of OPVs featuring Br-2PACz/MoO₃ – taken as the time over which the cell's PCE reduces to 80% of its initial value – increases compared to MoO₃-only cells from 297 to 615 h upon illumination and from 731 to 1064 h upon continuous heating. Elemental analysis of the BHJs reveals the enhanced stability to originate from the partially suppressed diffusion of Mo ions into the BHJ and the favourable distribution of the donor and acceptor components induced by the Br-2PACz.

A nonfullerene acceptor, isoIDITC, capable of exhibiting fibril-like morphology, is utilized as a third component in organic photovoltaic devices. A power conversion efficiency of 19% is achieved in ternary PM6:BTP-eC9:isoIDITC bulk-heterojunction devices.

Self‐assembled monolayers (SAMs) based on Br‐2PACz ([2‐(3,6‐dibromo‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid) 2PACz ([2‐(9H‐Carbazol‐9‐yl)ethyl]phosphonic acid) and MeO‐2PACz ([2‐(3,6‐dimethoxy‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid) molecules were investigated as hole‐extracting interlayers in organic photovoltaics (OPVs). The highest occupied molecular orbital (HOMO) energies of these SAMs were measured at −6.01 and −5.30 eV for Br‐2PACz and MeO‐2PACz, respectively, and found to induce significant changes in the work function (WF) of indium‐tin‐oxide (ITO) electrodes upon chemical functionalization. OPV cells based on PM6 (poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐fluoro)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b’]dithiophene))‐alt‐(5,5‐(1’,3’‐di‐2‐thienyl‐5’,7’‐bis(2‐ethylhexyl)benzo[1’,2’‐c:4’,5’‐c’]dithiophene‐4,8‐dione)]) : BTP‐eC9 : PC₇₁BM ([6,6]‐phenyl‐C71‐butyric acid methyl ester) using ITO/Br‐2PACz anodes exhibited a maximum power conversion efficiency (PCE) of 18.4 %, outperforming devices with ITO/MeO‐2PACz (14.5 %) and ITO/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT : PSS) (17.5 %). The higher PCE was found to originate from the much higher WF of ITO/Br‐2PACz (−5.81 eV) compared to ITO/MeO‐2PACz (4.58 eV) and ITO/PEDOT : PSS (4.9 eV), resulting in lower interface resistance, improved hole transport/extraction, lower trap‐assisted recombination, and longer carrier lifetimes. Importantly, the ITO/Br‐2PACz electrode was chemically stable, and after removal of the SAM it could be recycled and reused to construct fresh OPVs with equally impressive performance.

Doping is a common strategy in the field of semiconductor technology but its employment in organic photodetectors (OPDs) has been limited due to the typical uncontrollable increase of the dark currents. This study introduces three different molecular dopants, including p-type tris(pentafluorophenyl)borane, n-type benzyl viologen, and (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)-phenyl)dimethylamine, for near-infrared poly[[2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[3′,3′′-dimethyl-2,2′:5′,2′′-terthiophene]-5,5′′-diyl]:[6,6]-phenyl C₆₁ butyric acid methyl ester (PMDPP3T:PC₆₁BM) bulk-heterojunction OPDs. The results show that OPDs with optimal 0.02 wt% dopants exhibit low dark current (3.18 × 10⁻⁸ A cm⁻²), high detectivity (5.56 × 10¹² Jones), and good environmental stability for ∼2 months. These doped OPDs are further used for pulse wave monitoring, which exhibit stable waveforms and can distinguish slow and fast heartbeat rates.